The present invention relates to micro-analysis using X rays.
Numerous processes of sample analysis are known in which the sample is scanned by means of an electromagnetic or corpuscular radiation probe and the secondary radiation is detected. The complete analysis of a material requires in general several analyses of this material to be carried out by different processes, such as ESCA, AUGER, X-emission, X-absorption, X-fluorescence, etc. Several specimens are prepared from the same sample and, at the present time, it is necessary to use processes carried out in different apparatuses.
The present day processes involving analysis of a beam of X-rays have well known disadvantages: the count rates are low, the sensitivity when trying to detect the light elements is low (since the proportional counters, generally used as detectors, have a window which absorbs the soft X-rays) and the energy resolution of the X-photons is still higher than 5 eV in the best case.
Furthermore, conventional spectrometry of the X-photoelectrons (ESCA) does not allow exploration by scanning, for the area of the sample illuminated by the X-rays cannot be reduced below approximately 1 cm.sup.2. The area analysed in ESCA can however be reduced to a few .mu.m.sup.2, which allows exploration by scanning the sample to be analysed directly under the anticathode (J. Cazaux, Revue de Physique Appliquee, 10 (1975) p. 263). But this process is applicable only to the analysis of the sample in which the photoelectrons are created.
It is an object of the present invention to provide an improved process of micro-analysis using X-rays, answering better than those known heretofore the demands of practice.
If is a more precise object to provide a process which may be carried out in existing apparatus designed for implementing the ESCA or AUGER process.
According to an aspect of the invention, there is provided a process comprising the steps of forming an anticathode consisting of a thin layer of a material, scanning said anticathode with a thin beam of primary electrons to form X-rays, receiving said X-rays in a thin layer of a converter substance selected to have a photoelectronic spectrum which is simple and to exhibit one peak having a low bonding energy, collecting the current of photoelectrons resulting from absorption of said X-rays in the converter, measuring the instantaneous value of said current and determining the properties of the successive zones of the anticathode which receive the beam or the properties of zones of an absorbent layer placed between the anticathode and the converter.
To carry out a micro-analysis of the anticathode layer, the photoelectron beam is analysed by energy spectrometry and the distribution of a predetermined element in the anticathode is deduced from the intensity of a current of photoelectrons having an energy characteristic of this element. The energy resolution may be better than 1 eV and consequently makes it possible to determine the degree of oxidation of the element due to the chemical shift.
To measure the local absorption of an absorbent layer of a sample, placed between the anticathode and the converter, the photoelectrons may be collected, particularly when the sample is homogeneous, without discrimination in energy. A discrimination may be effected so that it measures the current of elastic photoelectrons due either to the radiation characteristic of the anticathode, or (at least in certain cases) to the fluorescence radiation emitted by the object. In both cases, a resolution of 10 eV is sufficient. If it is desired to analyse organic absorbent layers, an anticathode made from aluminum or magnesium will be advantageously used. In metallurgy, chromium will be used in preference. If the absorbent layer is formed by a light element, it will often be advantageous to form the anticathode from the element which follows the one forming the absorbent layer in the Mendeleen classification.
According to another aspect of the invention, there is provided a target for implementing the process defined above, comprising, forming a layer of a sample to be studied between a thick layer of a material forming the anticathode and a thin layer of a converter material. The sample may be organic (which will then lead frequently to sealing the edges of the anticathode and of the converter to form a sealed capsule protecting the sample) metallurgical or mineralogical. The material constituting the anticathode will be chosen with due consideration of the nature of the absorbent layer or that of the element to be detected therein.
The invention will be better understood from the following description of processes which form embodiments thereof given by way of examples. The description refers to the accompanying drawings.